1) Field of the Invention
The invention relates to a bitumen/polymer component capable of being employed for forming bitumen/polymer compositions with very low thermal sensitivity. It also relates to the bitumen/polymer compositions formed and to the use of the said component and composition for the production of surfacings and especially of road surfacings.
2) Background Art
One of the main causes of destruction of roadways made with the use of bituminous mixes prepared by employing bituminous binders is the loss of transverse smoothness as a result of the formation of ruts due to an irreversible creep of the bituminous binder forming part of the bituminous mix. In this process the characteristics of the bituminous binder become inadequate to maintain the cohesion of the structure and this results in a reduction, or even annulation of the void content of the bituminous mix layer. Under these conditions the bitumen acts as a lubricant rather than as a cohesion promoter. To overcome this disadvantage, which additionally presents obvious hazards where the safety of the road users is concerned, it has been proposed to increase the hardness of the bituminous binders employed, that is to say to make use of low-penetration bitumens as bituminous binders.
Another problem arises when large cracks appear in roadways during the first frosty days and above all when this frost reaches the deep courses of the roadway. Since the cooling of the surface structures of the roadway takes place more quickly than the cooling of the structures situated at greater depth, very large thermal shrinkage stresses are generated. Since the ability of a bitumen to withstand such stresses is directly related to its ability to relax these stresses, it is important that the bituminous matrix of a roadway made from bituminous mixes should have a high concentration of oily fractions. This entails the use of soft bitumens of high penetration to form the bituminous binder of the bituminous mix. The problem then lies in the ability of these bitumens to withstand the permanent deformations.
A bituminous binder which is suitable for the production of roadways must therefore have properties which represent the best possible compromise between two tendencies, namely brittle strength when cold, which requires the use of soft bitumens of high penetration to form the bituminous binder, and resistance to permanent deformations, which requires the use of bitumen of low penetration to constitute the bituminous binder.
These properties of a bituminous binder can be quantified by determining the plasticity range, defined as the difference between the ball-and-ring softening temperature (abbreviated to BRT), expressing the properties of the bituminous binder when heated, and the Fraass brittleness point, expressing the properties of the said binder when cold. The resistance of the bituminous binder to hot and cold stresses will be proportionally better the wider the plasticity range.
The penetration, ball-and-ring softening temperature (BRT) and Fraass brittleness point values employed for characterising the bituminous binders are obtained by standardised procedures (identified by French Standards) as shown below:
penetration: determined according, to French Standard NF standard T 66004 and expressed in 1/10 of mm; PA1 ball-and-ring softening temperature: determined according to NF standard T 66008 and expressed in .degree.C.; PA1 Fraass brittleness point: determined according to French Standard IP standard 80/53 and expressed in .degree.C.
The properties of a bituminous binder can also be quantified by determining the thermal sensitivity of the said binder from curves of variation of characteristics of the binder as a function of temperature, the thermal sensitivity being directly related to the slope of the said variation curves.
An indication of the thermal sensitivity of a bituminous binder can also be obtained from a correlation between the penetration (abbreviated to pen) and the BRT of the binder, known as the Pfeiffer number (abbreviated to PF).
This number is calculated from the relationship ##EQU1## in which A is the slope of the straight line represented by the equation ##EQU2##
The thermal sensitivity of the bituminous binder is proportionally lower the higher the value of the Pfeiffer number or--which amounts to the same thing--the lower the value of the quantity A.
In the case of conventional bitumens the Pfeiffer number assumes values which lie in the region of zero. Oxidised bitumens exhibit much higher Pfeiffer number values, of between 3 and 7, but their penetration is lower than 50 and can reach values as low as zero in some extreme cases, which means that these products will have an increased resistance to permanent deformations but will exhibit a very high brittleness when cold.
Bituminous binders of the bitumen/polymer type, especially of the bitumen/elastomer type or of the bitumen/plastomer type have Pfeiffer number values greater than zero and capable of reaching two, and this represents an important step forward towards the improvement of the performance of roadways subjected to increasingly severe stresses. However, this improvement still remains inadequate in the case of certain areas which are subjected to particularly severe stresses by very heavy traffic and in regions where the differences between summer and winter temperatures are very wide.